DESCRIPTION (Adapted from applicant's description): The goal of this project is to establish the pathophysiologic link between the mutation of the X chromosome and the phenotypic abnormalities in the inherited rickets. The investigators hypothesize that a Pi transport inhibitor, recently isolated from osteoblast conditioned media, in the laboratory is a potential candidate for the mediator of the disease. The goals of this project are to further purify and characterize the renal Pi transport factor, a potential candidate for the mediator of the disease with open column chromatography and successive HPLC steps. Upon achieving sufficient purity, its amino acid sequence will be determined. The sequence will be used to design appropriate primers for use in identification of relevant cDNAs from a murine total embryo cDNA library. Alternatively, PCR of bovine bone, or probing a cDNA library constructed from the investigator's osteoblast cultures will be employed. A major long-term goal is to establish a pathophysiologic link between the PEX mutation, humoral mediation, and the phenotypic abnormalities in the disorder. The findings should provide new insights in the biology of Pi homeostasis, and aid in the functional understanding of XLH.